In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed in key locations in the industrial process and coupled to control circuitry in a control room by a process control loop or segment. The term “field device” refers to any device that performs a function in a distributed control system or process monitoring system, including all devices used in the measurement control and monitoring of industrial processes.
Field devices are used by the process control and measurement industry for a variety of purposes. Usually, such field devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and be able to withstand climatological extremes of temperature, humidity, vibration, mechanical shock, etc. These field devices can also operate on relatively low power. For example, some field devices are able to operate solely on power that they received through a process communication loop or a segment sometimes with an operating current at or below 20 milliamps.
Some field devices measure a process characteristic and compute and transmit a process variable related to the measurement. These process variable transmitters can be used to provide information to the control room or monitoring station relative to temperature, pressure, flow, pH, turbidity, level, or any other suitable process variables.
In some instances, it may be useful to provide an indication of the process variable next to a final control device or to display information from one or more transmitters that are mounted in relatively inaccessible locations. In such instances, a field device known as a remote indicator is used. A remote indicator can be located anywhere along the process control loop or segment to allow information to be displayed wherever it is required. In some instances, the remote indicator can also perform basic arithmetic operations on the process variable(s) such that the indicated quantity is a calculated value.